Process of forming superficial alloys of chromium on metal bodies



Dec. l0, 1957 P. GALMICHE 1 2,816,048v

- PRocEss oF Femme SUPERFIGIAL ALLoYs Y OF CHROMIUM 0Nv METAL BODIESFileduarch 19, 195e /NVENTDR Phill' pf Galmcbe- ATTDHNE United StatesPatent O inociss's or Borrarme suPEnFiCIAL ALLoYs or CHROMIUM. oN METALnomas Philippe Galmiche, Paris, France, assigner to Olce NationaldEtudes` et `de `Recherches Aeronautiques, `Chatillon-sous-Bagrieux(Seine), France, a society of France AppicatinMrch 19, 1956,'Serial No.572,455 Claims priority, application France ugust'S, 1949l 1 1s' claimsi'(cl. 117-407) The :present invention relates to an improved `process offorming superficial alloys-ofchromium 'on metal bodiesandfmetalpwdersconsisting of or rhaving as a principal constituent ametal with which chromium -Will become lloyed under conditions promotinginter-metallic dif- Afusionof :the two metals.` The invention-hasparticular application-in thfe formation of supercial valloys of "chrovmium with? metal bodies' having asa principal'constituent a metalselected frornthev-group consisting of iron,lnickel and cobalt. -Otherapplications of `theinvention will be hereinafter described.

@The invention -in itsbroader aspects consistsin ysubl .(vapor tensionand boilingip'oint) andlits thermodynamic ,cgnstantsf are well adaptedto a balanced'exchange reaction inethe gaseous state, at thetemperatures necessary `forfthe-diliusion of chromium in the metal undertreatfeature" the step -ofg` transporting .the chromiumijluoride ,Y

vapors from the pointof `their generation, which may'be within or`outside Vthe chamber wherein the metal bodies lare disposed. fortreatment, Vto the treatment 'zone and maintaining the temperature insuch treatment vzone below the boiling point of thej fluoride but at apoint high enough to insure aconcentrationof the vapors sulicienttopro:- mote the intermetallic diffusion. In this way a `positive controlof the vapor concentration without loss of vapors to the system orbuilding up of undue. pressures l'within the treatment ,chamber isinsured.

Another feature of my invention relates moreV particuv larly towmethodsin.-which thepiecesto be treated are exposed tothe action of a fluorideof` the addition metal in the gaseous form and consists inproviding,.inthe reaction chamber, a reserve mass .of .the addition metal (or oneofitscompounds) intended directly to regenerate thefactive vapor.` Thismass may be `above the-.pieces to be treated or in contact therewith.

Still another feature consistsinperforminganelectrolyticlpolishinguafter the thermal treatment throughwhich a superiicial. alloy of diffusion was formed. on -the pieces to betreated. I `Preferred embodiments of my invention will `be hereinafterdescribed with reference to the accompanying drawings, given merely ybyway of example and infwhich the only ligurediagrammatically showsan-embodiment of an'apparatus for carrying out a method according to my'invention.

" My inventionwill be describedmore particularly as' applied-'to Athecase ofa method of forming asuperlicial l chromium alloy on steel pieces1.

2,816,048 latented Dee. 1o, 1957 1 According to my invention, I make useof chromium fluoride a's chromium carrier.

Preferably, pieces 1 are directly exposed to chromium fluoride vaporsformed and maintained during the operation, by the action of ammoniumuoride on chromium `or a Chromium compound.

Said vapors may also be obtained'by heating chromium iluoride preparedin advance and separated substantially from the substances which haveserved to its preparation.

"For instance, I may use commercial chromium fluoride, irrespective ofthe' fact that this uoride may contain some-amount of acid.

Advantageously I add to this chromium uoride some amount of the mixtureof ammonium uoride and chromium or a chromium compound. Both such amixture andc'liromiur'n liuoride formed inadvance will be' calledcementation product.

It "will be possible, Yby addition of complementary re- -Vagentsgsueh'for ins-tance as alumina, zirconia, aluminum,

silicon-or zirconium, to obtain mixed cementations.

" On the other` hand, I advantageously work in a reducing atmosphere,`for instance of hydrogen.

Chromium'transfer is then conditioned by the follow- 'ing'reaetionssFirst, there-is a disengagement of chromium and formation of ironfluoride.Vv

This'iron liuorideis immediately reduced by hydrogen `according 'to` thefollowing reaction: ,(21)

. Nlhechoice of chromium liuoride as a vehicle for the addition metalhas numerous advantages.

As a matterfof fact, the physical constantsof this-halide It should benoted that the use of a iiuorine compound `leads vto' the formation vof.an iron iluoride which, at the beginning of the ,operation and prior tothe iron reduction [see Reaction 2l constitutes a thin temporaryprotective layer which eliminates all risks of pitting of the treatedpieees,contrary to what takes place in particular with methodsmaking useofa chlorine compound which yields yolatileiron chloride, thus causingpitting which impairs the surfaces ofpieces treated in agaseous medium.

Y Another `advantage of the chromium uoride process lies in the factthat it is not hygroscopic so that when the dilusion chromium issupplied by means of a cementat1on product containing or constituted bychromium fluoride, :this cementation product can be handled after theoperation without giving rise to the formation of a stable hydratedhalide as is the case when the carrier element that is adopted ischromium chloride which releases vits water# of Acrystallization at ahigh temperature and causes the'as yet unreacted metallic chromium inthe treatment container to be oxidized.

A Finally, ammonium iiuoride is well adaptedto the application of aparticular feature of the invention according to which-water is added tothe cementation mixture in such manner as to permit initial attack onchromium in the liquid state.

` .It should be pointed out here that, in particular when itis desiredto operate in the presence of water, it will be possible eventually tosubstitute, either wholly or partly, hydrouoric-acd in solution for theammonium iiuoride entering into the composition of the cementationmixture.

`I thus get the advantage of a much higher coeflicient of utilization ofthe ammonium fluoride, that is to say of a higher amount of chromiumfluoride being formed. In the course of heating up, water is eliminated-and volatilizing of the excess of ammonium fluoride vdrives off thelast traces of steam so that, at the end of the treatment, there isobserved no oxidizing of the chromium present in the cementation mixturestill remaining in the reaction chamber. Furthermore, owing to thisaddition of water, the fumes of ammonium fluoride which are formed atthe beginning of the treatment are very substantially reduced.

Therefore, when using a cementation mixture I preferably adopt, as theinorganic fluorine compound of the cementation mixture, ammoniumfluoride (either in the neutral or in the acid state) which may beeither merely Ymoistened, or introduced in the form of an aqueoussolution, or again mixed with hydrouoric acid. p

Ammonium fluoride is the preferred source for the fluorine ion in thisinvention but hydrofluoric acid alone or various fluorine compounds,such as nickel fluoride or another suitable metallic fluoride, may beutilized. The primary consideration is that the iluorine compound.reacts with chromium or a chromium compound to form chromium fluoride.Although ammonium fluoride has been disclosed herein as producingchromium fluoride in situ, i. e., in the immediate vicinity of the metalto be treated, it will be understood that the invention may be practicedby utilizing chromium fluoride derived from other sources. For example,solid or liquid chromium fluoride may be positioned in the chamber inplace of the cementation mixture and vaporized by heating. Of course,this chromium fluoride in the solid or liquid state must not be incontact with the surface to be chromized. On the other hand, chromiumfluoride vapors may be generated and fed directly into the chamber froman outside source.

I will now indicate, by way of example, the reactions that take placeduring the formation of chromium fluoride from a cementation mixturewhich contains water.

During the attack in a liquid medium, the hydrofluoric acid resultingfrom hydrolysis of ammonium fluoride reacts upon chromium with theformation of chromium fluoride and hydrogen.

This reaction takes place chiefly at about 100 C. during the heating upof the reaction chamber which accordingly remains for a time at constanttemperature.

Ammonium fluoride then volatilizes, driving off the last traces of steamand partly Idissociating itself so as to form hydrofluoric acid which,upon reacting with chromium, yields chromium fluoride through the dryprocess in a non-oxidizing atmosphere. Partial dissociation of ammoniumfluoride takes place according to the following reaction:

.and the hydrofluoric acid reacts upon chromium according to thereaction:

Concerning the temperature of treatment, it will range from 600 to l200C. according to the nature of the metal that is being treated and to theduration of the operation, which duration may range from a fraction ofan hour to some tens of hours. For ferrous metals, it will be ofadvantage to maintain the temperature at from 1050 to ll C. Below about600 there will not be any appreciable diffusion of chromium into iron,nickel or cobalt. It is preferred to operate below the boiling point ofchromium fluoride (about 1300" C.) because the vapor pressure above theboiling point requires operation under higher pressures or,alternatively, provision for recirculation of the vapors.

When preparing a cementation mixture for use in practicing the process,chromium or the chromium compound (ferro-chrome for instance), after ithas been broken into pieces or powdered, is mixed with ammonium fluorideor another suitable fluorine compound and with a substance intended toavoid coalescence, that is to say agglomeraton by sintering of chromium.

Although I may consider utilizing, as such a substance, an inert orsubstantially inert matter such as kaolin, magnesia, hematite, I mayalso use a compound which participates in the reaction, which then makesit possible to obtain a mixed cementation intended, for instance. eitherto improve the resistance to oxidation (case of the addition ofalumina), or to insure a deeper penetration of chromium into ferrousmetals containing a high proportion of carbon (case of the addition ofzirconia).

Hereinafter, I will designate by the expression cementation agent thetotal mixture of chromium, ammonium fluoride and addition products. y

The decomposition of the cementation mixture gives a non-oxidizingatmosphere.

It is then possible, either to make use merely of this internalproduction, in which case it will be necessary to provide valves for theevacuation of the excess of gas without inflow of air, or artificiallyto supply the reaction chamber with a reducing or inert gas.

Concerning now the apparatus for carrying out such a method, I may makeuse, to constitute it, of the construction illustrated by the drawingand according to which;

There is provided at A, at the bottom of a cementation casing 2,preferably of vertical cylindrical shape, the

cementation agent as above defined;

Pieces 1 are separated from said agent by means of a partitioning 3, forinstance horizontal, these pieces being disposed above layer A, whichprevents their being in contact with the cementation agent.

Preferably, I provide, at the top of the cementation casing 2, a reserveB of chromium or ferro-chrome contained in a kind of basket 4 andserving to facilitate a direct regeneration of the active vapor.

The atmosphere in casing 2 is preferably a reducing atmosphere, forinstance of hydrogen supplied from the outside through a conduit 5.

The chemical process during heating is then as follows:

The chromium fluoride vapor reacts with iron according to Equation l,the chromium that is evolved being diffused into the pieces undergoingtreatment, whereas the iron fluoride is immediately reduced according toReaction 2 and regenerates iron which therefore does not leave thetreated piece; the hydrofluoric acid which is disengaged by thereduction of iron fluoride enters into contact with the reserve B ofchromium placedr at the top of the cementation casing 2 and a furtheramount of chromium fluoride is formed which, being heavier thanhydrofluoric acid, drops back onto the pieces in treat ment and yieldsthereto the chromium it contains.

rIhus a to and fro movement of the carrier fluoride takes place betweenthe cementation agent and the chromium reserve B, every passage on thepieces to be treated corresponding to a disengagement of chromium.

Furthermore, if alumina, zirconia, aluminum, silicon or zirconium isprovided in the cementation agent, a portion of the chromium compoundreacts on these cle-l ments to form the corresponding fluoride compoundwhich cooperates in the cementation process.

This method makes it possible to obtain, in particularly economicconditions, perfectly smooth and bright layers comparable with the bestelectrolytic deposits and involving no trace of adhesion of thecementation agent. The proportions of chromium are high in all cases andmay exceed 50% at the surface; by way of indication, the averageproportion of aluminum in the layers reaches about 2% in the case ofmixed chromium and aluminum cementation. A

As the treatment 'takes place ina vapor medium; it is possiblesuccessfully to treat pieces of complicated shape, involving recesses orholes.

By way of example, on mild steel containing 0.1 percent of carbon, itis" possible tob'tai t- 1-100' G5.- a depth of penetration of 0.2millimeter in an operation of a duration of four hours, at l000 C. apenetration of 0.1 mm. in the same time. The hardness of the layerapproximates that of iron-chromium alloys, i. e. isclose to 200 Vickers.The layers are lxible and ductile and therefore have good characteristis'f rsistance to wear and tear; they have a high resistance to corrosionby nitric acid and solutions of sodium chloride; at the same time, theyhave a good resistance to dry oxidation during very long times up totemperatures above 900-l000 C.; their regularity and their homogeneityare perfect.

On steels containing a higher amount of carbon, the

thickness that is obtained is smaller but the hardness higher; by way ofexample, with steel containing 0.5% of carbon, there is obtained a layerof 0.03 mm. in four hours at l000 C., the hardness averaging 800Vickers. If an initial decarburizing of the steel on cast iron isperformed, for instance by heating in moist hydrogen, it is possible toobtain, on steels containing a high amount of carbon, thicknessesequivalent to those obtained on mild steels.

Another embodiment yof my invention consists in heating pieces 1 incontact with chromium or ferro-chromium broken into pieces and belowwhich the cementation mixture is placed.

It should be noted that, in all cases, the treatment might be conductedin an ammonia atmosphere or, still better, be followed by a nitridingtreatment by ammonia at temperatures ranging from 600 to l000 C. Thiswould make it possible to obtain layers of an extremely high hardness,as high as 1500 Vickers, having on the other hand good friction andwearing properties and also a high resistance to corrosion and dryoxidation.

The treatment above described may be applied in the same conditions toother metals or alloys such as nickel, cobalt, high nickel-molybdenumiron-alloys known under the name Hastelloy, either sintered or in powderform.

Finally, it will always be possible, according to a feature of theinvention applicable to all thermal chromium coating method in a gaseousmedium but more particularly advantageous in the case of iiuoridecementation, to subject the pieces that have been chromium coatedthrough such methods to a subsequent anodic polishing treatment whichpermits of obtaining, in addition to the brightening inherent in suchpolishing treatments, elimination of the sigma state of theferro-chromium, which has a tendency to appear, in certain conditions,at the outer limit of the diffusion layers, such a state having thedrawback of lowering the resistance of the surface to said corrosion.

For this purpose, it will be advantageous to have recourse to an anodicpolishing process making use of an electrolytic bath containingperchloric acid and acetic acid.

In a general manner, while I have, scription, disclosed what I deem tobe practical and efficient embodiments of my invention, it should bewell understood that I do not wish to be limited thereto as there mightbe changes made in the arrangement, disposition and form of the partswithout departing from the principles of the present invention ascomprehended within the scope of the accompanying claims.

This is a continuation-in-part application of my copending application,Serial No. 175,502, filed Iuly 24, 1950, now abandoned.

I claim:

1. A process of forming a supercial alloy of chromium on a body havingas a principal component a metal selected from the group consisting ofiron, nickel and cobait comprising heating a cementation mixturecontaining in the above desaid heating treatment;Y

acrobate chromium and an inorganic fluorine compound capable ofreacting. therevs'fithl to form chromium fluoride, bsaid heating, beingcarried out at a'- t'emperature to produce vapors of sai'd chromiumlluoride, and bringing: said chromium fluoride vapors inthe presence or'hydrogen into contact with said metal'body at a tenrperatnreabove 600'C. while maintainirigsaid cementation mixture out of contact with saidmetal body.

2. A process as claimedin`l claim l whereinsaid cementation mixtureycontainsV water at the beginning of 3. A process of forming asuperficial alloy of chromium on a body having as a principal componenta nieta-l selected from the group consisting of iron, nickel and cohaltcomprising positioning said body in a chamber, positioning in saidchamber spaced from said body a cementation mixture including chromiumand an inorganic fluorine compound capable of reacting therewith to formchromium fluoride, heating said chamber within the range of 600 to 1300uC. to form said chromium iluoride and produce vapors thereof, andbringing said vapors in the presence of hydrogen into contact with saidbody.

4. A process as claimed in claim 3 wherein said inorganic uorinecompound is hydrouoric acid.

5. A process as claimed in claim 3 wherein said inorganic iluorinecompound is ammonium liuoride.

6. A process as claimed in claim 5 wherein said metal is iron.

7. A process as claimed in claim 3 wherein a further mass of chromium ispositioned in said chamber and spaced above said mixture.

8. A process as claimed in claim 7 wherein said further mass of chromiumis spaced above said body.

9. A process as claimed in claim 7 wherein said further mass of chromiumis in contact with said body.

10. A process of forming a superficial alloy on a body having as aprincipal component a metal selected from the group consisting of iron,nickel and cobalt comprising positioning said body in a chamber,positioning in said chamber spaced from said body a cementation mixtureincluding chromium, an inorganic fiuorine compoundcapable of reactingtherewith to form chromium iiuoride and a material selected from thegroup consisting of alumina, aluminum, zirconia, zirconium and silicon,heating said chamber within the range of 600 to 1300" C. to react saidmixture components and produce vapors, and bringing said vapors in thepresence o hydrogen into contact with said body, whereby an alloy isformed on said body.

1l. A process as claimed in claim l0 wherein said metal is iron and saidfluorine compound is ammonium fluoride. l

l2. A process as claimed in claim l0 wherein said second compound isalumina.

13. A process of forming a superficial alloy of chromium on a surface ofa body having as a principal component a metal selected from the groupconsisting of iron. nickel and cobalt comprising bringing chromiumiluoride in the state of vapor in the presence of hydrogen into contactwith the whole of said surface while maintaining a temperature of above600 C.

14. A process as claimed in claim 13 whrein said temperature ismaintained below the boiling point of chromium fluoride.

15. A process of forming a superficial alloy of chromium on a bodyhaving as a principal component a metal selected from the groupconsisting of iron, nickel and cobalt comprising positioning said bodyin a chamber, heating chromium uoride prepared in advance and separatedsubstantially from the substances which have served to its preparationto produce chromium uoride vapors, and bringing said chromium uoridevapors in the presence of hydrogen into contact with said metal body ata temperature above 600 C. while maintaining said chromium fluoride outof contact with said metal body.

2,816,048 7 16. A process as claimed in claim 15 wherein a furto 1300 C.to form chromium fluoride vapors, and bringther mass of chromium ispositioned in said chamber.

ing said vapors in the presence of hydrogen into Contact with said body.

I References Cited in the file of this patent UNITED STATES PATENTScobalt comprising posltlonng Said body 1n a Chamber ggg? "Ig/[113epositioning in said chamber spaced from Sald body a 2219004 Daeve Qt-1-n se i 30 1941 cementation product essentially cOnStlUfSd by CbfOmlum 102,257,668 B k S t 1 u S pt' 30 194' Hum-ide, heating said Chamber Withinthe fange 0f 600 ec ere a ep g l

13. A PROCESS OF FORMING A SUPERFICIAL ALLOY OF CHROMIUM ON A SURFACE OFA BODY HAVING AS A PRINCIPAL COMPONEBT A METAL SELECTED FROM THE GROUPCONSISTING OF IRON, NICKEL AND COBALT COMPRISING BRINGING CHROMIUMFLUORIDE IN THE STATE OF VAPOR IN THE PRESENCE OF HYDROGEN INTO CONTACTWITH THE WHOLE OF SAID SURFACE WHILE MAINTAINING A TEMPERATURE OF ABOVE600*C.